Shares in BlackBerry, the Canadian smartphone maker which maintains a R&D operation in the Triangle, fell the most in two months after analysts said sales of its new Z10 device are showing troubling signs. The company denied one of the reports.

BlackBerry (Nasdaq: BBRY) slid 7.8 percent to $13.55 at the close in New York, the biggest decline since Feb. 13.

“The U.S. launch of the Z10 started poorly and weakened significantly as the days passed,” Joseph Fersedi, an analyst at ITG Investment Research, said today in a note, citing information from independent dealers. Some U.S. retailers are seeing a significant increase in customers returning their Z10s because they find the interface unintuitive, Detwiler Fenton & Co. said Thursday.

“In several cases, returns are now exceeding sales, a phenomenon we have never seen before,” Detwiler Fenton said.

BlackBerry called Detwiler Fenton’s claim “absolutely false,” according to an e-mailed statement. “Our data shows that return rates for BlackBerry Z10 devices both in the U.S. and on a global basis are in line with or better than our expectations and are consistent with return rates for other premium smartphones in the market today,” it said.

The company is counting on the touch-screen Z10, its first smartphone to use the BlackBerry 10 platform, to revive growth after losing ground to Apple Inc.’s iPhone and devices that use Google Inc.’s Android software. Retailers in the U.K. and Canada are taking pre-orders for a second BlackBerry 10 phone, the Q10, which has a physical keyboard.

T-Mobile Debuts $99 IPhone  

Also on Thursday, T-Mobile USA Inc., the fourth- largest U.S. wireless carrier, will begin offering Apple Inc.’s iPhone for the first time today, providing the biggest showcase yet for its new installment-plan approach to selling phones.

Customers with good credit can buy the iPhone 5 for $99.99 down and 24 monthly payments of $20, the Bellevue, Washington- based company said, breaking from a tradition of subsidizing smartphones in exchange for two-year service contracts. T-Mobile also will take old iPhones as trade-ins, in exchange for a new iPhone 5 with no down payment and a credit toward future bills.

The strategy “could resonate with customers,” Walt Piecyk, an analyst with BTIG LLC, said this week in a research note. At about $100, the upfront costs for an iPhone 5 through T-Mobile are lower than the $199 typically charged by rivals.

T-Mobile, a division of Deutsche Telekom AG, is counting on the Apple device to help reverse an exodus of subscribers. The company was the last of the four major U.S. carriers to get the iPhone, and T-Mobile has trailed competitors in adopting a speedier network standard called long-term evolution, or LTE.

As part of a makeover under Chief Executive Officer John Legere, the company is offering no-contract service plans that start at $50 a month. He has begun calling T-Mobile “the uncarrier” in an effort to distance itself from its three larger rivals, which still rely mostly on long-term service commitments. It also aims to bolster its network and customer base through a merger with MetroPCS Communications Inc., a deal brokered by parent Deutsche Telekom.

BlackBerry Phone Offer

T-Mobile is offering BlackBerry’s new Z10 phone using a similar installment plan, and it plans to begin selling Samsung Electronics Co.’s Galaxy S4 on May 1.

“Our goal is to give customers the lowest out-of-pocket cost of anyone in the industry,” Legere said when he announced the iPhone plan last month. “We’ll see how the competition responds. I don’t think the math is going to work out for them.”

T-Mobile lost 2.1 million monthly contract customers last year. Legere, who took the reins in September, has said the addition of the iPhone — along with the new pricing plans and network upgrades — will allow T-Mobile to stem subscriber losses in 2014.

Even after combining with MetroPCS, T-Mobile will remain a distant No. 4 in the U.S. market to Verizon Wireless, AT&T Inc. and Sprint Nextel Corp.

The combination of T-Mobile and MetroPCS would create a company with 42.3 million customers, with Deutsche Telekom owning 74 percent. The German phone company sweetened the terms of the deal this week to placate investors. The new offer, which now has the support of MetroPCS’s largest shareholder, will go to a vote on April 24.